This invention relates to a process for separating C.sub.2+ hydrocarbons from high pressure natural gas by rectification wherein the temperature and pressure values required for rectification are obtained by heat exchange and expansion, and wherein the resultant condensates from the expansion are introduced into a rectifying column.
A process of this type is described in U.S. Pat. No. 3,292,380. In this process, the natural gas is first cooled by heat exchange with cold rectified product, thus obtaining the readily condensible components in the liquid phase; a portion of the condensate is expanded and then introduced into the rectifying column. The components of the natural gas remaining in the gaseous state after said heat exchange are engine-expanded in an expansion turbine to the pressure of the rectifying column, thereby providing refrigeration for the process.
Frequently, cooling in heat exchange with cold product gas is insufficient so that additional external refrigeration is required, such as, for example, a propane or propylene cycle. In such a refrigerant cycle, losses of refrigerant occur, which in turn must be compensated for by adding more refrigerant. Another disadvantage of the conventional process is that it cannot be utilized with natural gas that contains significant quantities of carbon dioxide unless the CO.sub.2 is separated from the natural gas in a special preliminary process step. Otherwise there would be the danger that solid precipitants are formed in the parts of the apparatus operated at low temperatures and/or with high CO.sub.2 concentrations, and in some cases even complete blockage of conduits, etc. resulting in shutdown could occur. Finally, whereas the use of an expansion turbine is conventional, it is expensive equipment both with respect to initial investment and operating costs.